This invention relates to a drive and brake system for a vehicle having a constantly driven rear axle and a front axle controllably driven via an engageable and disengageable front wheel drive clutch.
In one commercially available tractor, it is known to provide a control system which automatically engages its 4-wheel drive when braking and which automatically disengages the 4-wheel drive when the tractor ground speed is greater than 9 miles per hour. However, this known control system does not automatically disengage the 4-wheel drive when only one of the left and right brakes is applied. This is undesirable because when operating an agricultural tractor, it is often advantageous to use only one of the left or right foot brakes to assist in steering the tractor through a headland turnaround. Having the 4-wheel drive engaged during such a brake-assisted steering operation interferes with such turning in that more brake effort is required and front tire pushing, skidding and wear is increased.
From published German patent DE No. 2,946,477, it is known to engage a front-wheel drive clutch which transmits power to a front axle when service brakes for the rear axle are applied. Thus, in this system, it appears that the front wheel drive can interfere with brake-assisted steering because there is no suggestion that the front wheel drive is not engaged when only the left or the right wheel brake is applied. In one embodiment described in DE No. 2,946,477, a solenoid-operated valve controls a pressure-operated front wheel drive clutch and the solenoid energization is controlled by a brake-responsive switch. However, the system of DE No. 2,946,477 requires a distributor valve hydraulically coupled to left and right brake cylinders and a piston hydraulically connected to the distributor valve and mechanically connected to the brake-responsive switch. Thus, such a system requires expensive hydraulic components which rob hydraulic fluid that might be needed for emergency braking.
U.S. Pat. No. 4,811,811 issued March 4, 1989 and U.S. Pat. No. 4,878,559, issued Nov. 7, 1989, both assigned to the assignee of the present application describe control systems which control the engagement and disengagement of a tractor front wheel drive clutch as a function of vehicle speed and as a function of an "AUTO-ON-OFF" operator controlled mode switch.
In one of the control systems disclosed in these references, even though the mode control switch is in the "OFF" mode, the clutch engagement will vary as a function of brake application at both high and low speeds. In the other control systems disclosed therein, when the mode control switch is in the "OFF" mode, the clutch will be disengaged regardless of vehicle speed or brake application. It is desired to have such a control system with an "OFF" mode wherein the clutch is always disengaged at low vehicle speeds, but wherein four wheel braking would be available at high vehicle speeds